The disclosed methods relate to methods of treating disorders through transplantation of cells that are uniquely beneficial for such treatment methods. In particular, the disclosed methods provide methods of treating neurodegenerative conditions with neural stem cells (NSCs).
Neurodegenerative disorders are characterized by conditions involving the deterioration of neurons as a result of disease, hereditary conditions or injury, such as traumatic or ischemic spinal cord or brain injury.
The circuitry of the spinal cord that governs contraction of skeletal muscles of the limbs, involves excitatory motor neurons and inhibitory GABAergic (i.e., GABA-producing) and glycinergic (i.e., glycine-producing) inter-neurons. A motor neuron is a nerve that originates from the anterior horn of the gray matter of the spinal cord. The axon of the motor neuron emerges from a segment of the spinal cord as an efferent motor fiber that innervates muscle fibers. Impulses conducted by the motor neuron stimulate the muscle fibers to contract. GABA, gamma-amino butyric acid, is a naturally occurring metabolite of the mammalian nervous system which acts as a neurotransmitter to inhibit or dampen the nerve conduction of electrical potential. Loss of GABAergic interneurons results in dysregulation of the inhibitory tonality of the motor neuron-evoked muscle contractions. Without the control exerted by inhibitory interneurons on excitatory neurons, an over-firing of excitatory neurons occurs leading to spastic uncontrolled contraction or uncontrolled rigidity of the muscles of the limbs. Loss of motor neurons results in flaccid paraplegia in which the subjects cannot contract the muscles and are thereby unable to move.
One instance in which GABAergic interneurons are damaged in the spinal cord includes a complication associated with a transient cross-clamping of the descending thoracic or thoracoabdominal aorta. Such cross-clamping is a necessary step in vascular surgery to repair aneurysms of thoracic or thoracoabdominal aorta. For the duration of the cross-clamping, a portion of the spinal cord does not receive blood circulation and can become ischemic. Depending on the duration of the ischemic interval, subsequent neurodegenerative dysfunction may be expressed neurodegeneratively as paraparesis or fully-developed spastic or flaccid paraplegia.
While the mechanism leading to ischemia-induced neuronal degeneration is only partially understood and may involve excessive release/activity of excitatory amino acids, prostaglandins and/or oxygen free radicals, the neuronal population of spinal cord affected by transient ischemic insult are well defined. For example, histopathological analysis of spinal cord taken from animals with fully developed spastic paraplegia shows a selective loss of small inhibitory neurons; however, alpha-motoneurons persist in previously ischemic spinal segments. Similar spinal neuronal pathology in human subjects having spinal ischemic injury has been described.
In contrast, in animals with flaccid paraplegia, pan-necrotic neurodegenerative changes are seen affecting both small inhibitory and excitatory interneurons as well as ventral motor neurons. During the period of neuronal degeneration after spinal ischemia, an injury-dependent activation of local microglia and inflammatory changes, such as infiltration with macrophages, is also seen as in focal or global brain ischemia. Depending on the extent of injury, the inflammatory changes typically peak between two to seven days after ischemic insult and then show gradual loss of inflammatory elements over two to four weeks of post-ischemic period.
In the past two to three decades, a considerable effort has been made to assess in animal models the therapeutic potential of spinal grafting of a variety of materials. Thus, cell lines or acutely isolated spinal cord fetal tissue have been delivered to injured regions and direct spinal gene therapy has also been used to ameliorate neurodegenerative dysfunctions in several models of spinal injury, including mechanical traumatic injury, chemically lesioned spinal cord or genetically manipulated animals with progressive α-motoneuronal degeneration (ALS transgenic mice or rat).
In general, studies demonstrate long-term survival and preservation of neuronal phenotypes in grafts generated from fetal tissue, but not from neural precursors that have been expanded in vitro. In fact, only limited neuronal differentiation and maturation of neural precursors expanded in vitro and grafted into mechanically or chemically injured spinal cord has been demonstrated. Cells preferentially differentiate into non-neuronal cell types. While the mechanism of this preferential non-neuronal differentiation is not completely understood, it is hypothesized that a local release of pro-inflammatory cytokines (such as TNFα, TGFβ at the site of previous injury is likely involved.
Neurodegeneration represents a particularly challenging biological environment for cell therapy and cell death signals present in established neurodegenerative disease (Rothstein et al., 1992; Howland et al., 2002; Turner et al., 2005) may be incompatible with graft survival. In addition, the adult spinal cord is viewed as lacking cells and/or signals allowing regeneration (Park et al., 2002), and the majority of NSC grafting studies have shown poor or restricted differentiation (Cao et al., 2002; Yan et al., 2003; Yan et al., 2004).
One of the major problems in cell therapeutics is low cell survival (less than 5%) of the cells grafted. All of the grafted cells to date undergo significant cell death shortly after injection in vivo. Thus, in order to deliver an effective dose of cells, the final dose must be injected at least 20 times. This, in turn, requires a much larger scale of cell manufacturing which poses further regulatory and economic obstacles. Furthermore, the survival rate of such cells in vivo has not been able to be maintained. Failure to demonstrate reproducible administration of effective doses of cell therapy prevents approval for use by government and other regulatory agencies such as the Food and Drug Administration.
Additional challenges are presented when treating neurodegenerative diseases and conditions that are disseminated over a large area of a body, tissue, or organ, such as the entire nervous system rather than a single localized area. For example, in ALS, neurodegeneration involves slow death of motor neurons along the entire spinal cord as well as those neurons in motor cortex. Likewise, in most lysosomal diseases, neuronal destruction involves most regions of the brain and spinal cord. Alzheimer's disease involves most of the cerebrum. Even in more localized neurodegenerative diseases such as Parkinson's and Huntington's, the affected area of striatum is quite large, much larger than the grafting area that can be surgically reached. Thus, cell therapeutics for neurodegenerative diseases are expected to require wider grafting procedures.
There is, therefore, a need for improved methods of treating neurodegenerative conditions. There is also a need for improved methods of culturing and transplanting human neural stem cells and human neural progenitors that once grafted overcome all of the previously seen limitations and provide functional benefit. Thus, this method of treating neurodegenerative conditions, in vivo, generates robust neuronal differentiation, permits long-term neuronal survival under various degenerative conditions and maturation into therapeutically relevant subpopulations of neurons in adult tissues that lack developmental cues, and provides wide therapeutic range than the location of the cells themselves.